1. Field of the Invention
This invention relates to a bandwidth compression system and method and more particularly relates to a bandwidth compression system and method for converting a video signal into a narrow-bandwidth signal suitable for transmission.
2. Description of the Prior Art
It is often times desirable that a wide bandwidth signal be converted to a narrow bandwidth signal so that the resulting signal can be more easily handled such as, for example, in signal transmission. While such a signal might be transmitted over wide bandwidth lines, this has been found to be undesirable since such transmission is expensive. It has therefore been found desirable to convert wide bandwidth video signals, such as real-time television signals, for example, to narrow bandwidth signals so that the necessary information can be transmitted over less expensive transmission lines, such as telephone circuits, for example, as compared with coaxial cables, microwave systems and the like which are required for wide bandwidth transmission.
It has been found heretofore that narrow bandwidth video signals can be generated using special cameras or scanning devices having very slow line rates and frame rates. Known apparatus and methods utilizing these techniques, however, have not been found to be completely satisfactory, at least for some purposes and at least some such known devices have had undesirable limitations from an operational standpoint, including problems relating to equipment availability, monitoring equipment and techniques, stability, and signal-to-noise ratios.
A different approach to the generation of narrow-bandwidth video signals has employed sampling techniques wherein pictorial information signals are generated in the camera or scanning device running at real-time rates and the bandwidth reduced by sampling the video signal at a much lower rate. Such a technique has been described, for example, by Altes and Reed in an article entitled "Slow-Scan Adapter for Conventional TV Signals" in the June 1, 1957 issue of Electronics Magazine. In addition, such techniques are also described in U.S. Pat. No. 2,955,159 issued to C. E. Jones, Jr., and entitled "NARROW BAND VIDEO COMMUNICATION SYSTEM", U.S. Pat. No. 3,284,567 issued to Glen R. Southworth and entitled "NARROW BANDWIDTH TELEVISION SYSTEM" and U.S. Pat. No. 3,683,111 issued to Glen R. Southworth and entitled "TELEVISION BANDWIDTH COMPRESSION AND EXPANSION SYSTEM".
While apparatus and methods utilizing sampling techniques have proved to be useful at least for some applications, it has been found that at least some such systems and methods have not proved to be completely satisfactory for all intended purposes and have, for example, been found to: require camera modification; be restricted to an output data rate that is the same as the sampling rate; require a dot-interlaced sampling in order to obtain bandwidth compression to telephone circuit bandwidths which in turn creates a problem of non-compatability with other existing reproducing systems, requires special synchronizing circuitry at the receiver, and is subject to inter-symbol interference which when caused by transmission circuits results in multiple "ghosting"; and/or have an inability to provide a non-interlaced continuous bandwidth output signal while utilizing the information from both fields of an interlaced input signal.